There are no hard and fast rules, and of course it depends when you are talking about, but going back a 100 years or so I think it was usual for the first born son to be named after his father, and the first daughter to be named after her mother. Then it varies a bit... the second son may be named after the father-in-law, but he may also be named after an uncle, perhaps because in some families if 1st sons are John then 2nd sons are always Thomas etc.